1. Field of the Invention
This invention relates generally to a method and apparatus for improving the quality of photographic images and, more particularly, to improving digital images by selectively adjusting the brightness of large regions in such images without adversely affecting other regions regardless of size by leaving artifacts at the boundaries.
2. Description of the Prior Art
Real complex scenes have a luminance dynamic range that spans four decades. Hardcopy output media, however, are limited to about two decades. Humans can distinguish image features over the wide range of luminance in real scenes by an adjustment process which is not entirely understood but which is, nevertheless, observable. Various approaches have been attempted to compensate for the limitations of hardcopy media. The preferred conventional solution to the problem is known as tone compression of input scene luminance by means of a so called H and D curve. The approach essentially clips or reduces the visibility detail in dark and light areas and properly renders the midtones. The method inherently results in the loss of information at the extremes of the compression curve known as the toe and shoulder. Some variation of image quality is possible by adjustment of where in the curve the image is developed. This may be accomplished by means of filters and adjustment of print exposure time or intensity.
Unfortunately, the conventional method may result in lost information and reduced picture quality when the scene has extremely bright or dark areas. For example, scenes with a large shaded area and an adjacent bright or sunny area are especially difficult to develop or display without loss of some image quality in either or both of the dark and bright regions. Conventional methods for example preserve edge contrast at the boundary between large dark areas and large bright areas. The resulting adjustment leaves an artifact in the form of a halo at the boundary which resembles misregistration.
While adjustment of the exposure aperture setting, the time of exposure and the employment of filters can improve the image, it is not possible to efficiently bring up or brighten the dark areas without adversely affecting the light areas and vice versa. Also, it is not possible to effectively distinguish large areas which require adjustment from small areas such as image details or the like which do not require any appreciable adjustment. Further, the methods which have been employed are essentially optical or mechanical systems and not electronic systems.
The causes of diminished image quality are equally applicable in systems employing electronic processing of still photographic images. Its with this form of photography that the present invention is primary concerned. It should be understood, however, that the invention is useful with other forms of image processing. The invention however is not designed to correct for exposure errors. It assumes image details in the midtones is properly rendered.